1. Field of the Invention
The present invention generally relates to an optical scanning device and an image-forming device, and more particularly, to an optical scanning device and an image-forming device using this optical scanning device, such as a laser beam printer, a facsimile using plain paper, or a digital copying machine.
2. Description of the Related Art
Conventionally, there has been a well-known optical scanning device in which a light beam is reflectively deflected so that the light beam scans a scanned surface. Specifically, this optical scanning device reflectively deflects a laser light by using an optical deflector so that the laser light scans the scanned surface, and at the same time, turns on and off the laser light by using an image signal so as to write an image on the scanned surface.
A revolving polygon mirror revolving at a constant velocity is widely used as the above-mentioned optical deflector. However, the revolving polygon mirror requires a large-scale device. Additionally, the revolving polygon mirror involves a mechanical high-speed revolution, and thus causes such problems as a bounding due to vibrations, a temperature rise, noises, and an increase in the amount of consumed electric power.
On the other hand, there has been proposed a micro mirror having a resonant structure, using a micro-machine technology, which oscillates in a sine-wave form. Using this micro mirror in an optical scanning device can make the optical scanning device small, and can reduce the above-mentioned bounding due to vibrations, the temperature rise, the noises, and the amount of consumed electric power. Japanese Laid-Open Patent Application No. 8-75475 describes an optical scanning device employing a deflection mirror utilizing a resonance as mentioned above.
An optical scanning device is generally required to control a light beam reflectively deflected by an optical deflector in such a manner that the light beam scans a scanned surface at a substantially constant velocity, and also is required to have a predetermined effective write width.
Such an optical scanning device employing a deflection mirror utilizing a resonance as mentioned above is also required to control a light beam reflectively deflected by the deflection mirror oscillating in a sine-wave form so that the light beam scans a scanned surface at a substantially constant velocity, and at the same time, is required to obtain a large effective write width.
It is a general object of the present invention to provide an improved and useful optical scanning device and an image-forming device in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an optical scanning device and an image-forming device which can obtain a large effective write width, and can provide an excellent constant-velocity scanning property on a scanned surface.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an optical scanning device comprising:
a light source;
a deflection mirror reflectively deflecting a light beam projected from the light source by performing a sine-wave oscillation; and
a scanning optical element causing the light beam reflectively deflected by the deflection mirror to scan a scanned surface at a substantially constant velocity,
wherein the following conditional expression is fulfilled:
0.4 less than xcfx86max/xcfx860 less than 0.9/[(20/xcfx860){circumflex over ( )}(xc2xc)], 
where:
xcfx860 is an amplitude angle of the sine-wave oscillation of the deflection mirror, the amplitude angle being represented by degrees; and
xcfx86max is a maximum rotational angle of the deflection mirror corresponding to an effective write width, the maximum rotational angle being represented by degrees.
According to the present invention, using the deflection mirror reflectively deflecting a light beam by oscillating in a sine-wave form can alleviate a bounding due to vibrations, a temperature rise, noises, and the amount of consumed electric power. Also, fulfilling the foregoing conditional expression can provide a large effective write width, and can realize an excellent constant-velocity scanning property on a scanned surface.
Additionally, in the optical scanning device according to the present invention, the following conditional expression may be fulfilled:
0.5 less than xcfx86max/xcfx860 less than 0.8/[(20/xcfx860){circumflex over ( )}(xc2xc)]. 
According to the present invention, fulfilling this conditional expression can provide a larger effective write width, and can realize a more excellent constant-velocity scanning property on a scanned surface.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention an optical scanning device comprising:
a light source;
a deflection mirror reflectively deflecting a light beam projected from the light source by performing a sine-wave oscillation;
a reflective surface placed opposite the deflection mirror so as to reflect the light beam reflectively deflected by the deflection mirror to the deflection mirror; and
a scanning optical element causing the light beam to scan a scanned surface at a substantially constant velocity, the light beam being reflectively deflected by the deflection mirror at least twice,
wherein the following conditional expression is fulfilled:
0.4 less than xcfx86max/xcfx860 less than 0.9/[(20/(xcfx860xc3x97M)){circumflex over ( )}(xc2xc)], 
where:
xcfx860 is an amplitude angle of the sine-wave oscillation of the deflection mirror, the amplitude angle being represented by degrees;
xcfx86max is a maximum rotational angle of the deflection mirror corresponding to an effective write width, the maximum rotational angle being represented by degrees; and
M is a number of times the light beam is reflectively deflected by the deflection mirror.
According to the present invention, using the deflection mirror reflectively deflecting a light beam by oscillating in a sine-wave form can alleviate a bounding due to vibrations, a temperature rise, noises, and the amount of consumed electric power. Additionally, the light beam is reflected between the reflective surface and the deflection mirror several times, i.e., the light beam is reflected by the deflection mirror a plurality of times, so as to obtain a large effective write width. Further, fulfilling the foregoing conditional expression can provide a still larger effective write width, and can realize an excellent constant-velocity scanning property on a scanned surface.
Additionally, in the optical scanning device according to the present invention, the following conditional expression may be fulfilled:
0.5 less than xcfx86max/xcfx860 less than 0.8/[(20/(xcfx860xc3x97M)){circumflex over ( )}(xc2xc)]. 
According to the present invention, fulfilling this conditional expression can provide a still larger effective write width, and can realize a more excellent constant-velocity scanning property on a scanned surface.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention an image-forming device comprising at least one of the above-mentioned optical scanning devices causing a light beam to scan a scanned surface of an image-bearing member so as to form an electrostatic latent image on the scanned surface of the image-bearing member.
According to the present invention, by using the optical scanning devices employing the above-mentioned deflection mirror, an image with an excellent quality can be formed.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.